Hot rolled steel strip



United States Patent 3,245,844 HOT ROLLED STEEL STRIP Anthony P. Weber, Grifiith, Ind., assignor to United States Steel Corporation, a corporation of New Jersey No Drawing. Filed July 25, 1962, Ser. No. 212,464 Claims. (Cl. 148-12) This invention relates generally to hot rolled steel strip,

' and more particularly to the production of high strength steel strip from carbon steel having increased nitrogen content.

It is a principal object of this invention to provide a relatively inexpensive high strength steel which does not require costly alloy additions.

A related object of this invention is to provide an inexpensive plain carbon steel having increased nitrogen content, and control the hot rolling and coiling temperature during processing to provide high strength steel strip.

Yet another object of this invention is the provision of a method of producing a steel with high yield strength from plain carbon steel having a high nitrogen content.

These and other objects will become apparent from the following descriptions and claims.

I have found that by providing a relatively inexpensive low carbon steel having increased nitrogen content, and by properly controlling the finishing temperature on the hot rolling mill and the coiling temperature a steel strip having a yield strength of at least 50,000 p.s.i. can be obtained. By utilizing a low carbon steel differing only in nitrogen content from conventional carbon steel, a relatively inexpensive steel, properly processed, possesses the high strength properties in the hot rolled condition which have been available heretofore only in more expensive steels having costly alloy additions.

According to this invention a steel blank, having the following preferred composition, is provided: C, .16 to .20%; Mn, .40 to 58%; N, .010 to .013%; S, .040% max.; P, .020% max.; Si, .020% max.; Fe and residual impurities, balance.

The blank is heated to hot rolling temperature and then conventionally hot reduced to steel strip on a multiple stand hot rolling mill. It is necessary that the blank be heated to a sufficiently high temperature so that the temperature of the strip as it emerges from the final stand, i.e., the finishing temperature, is at least 1520 F.

After the strip emerges from the final stand it is rapidly cooled to below 1050" F. and then coiled on a conventional coiler. It has been found that the preferable way to rapidly cool the strip to below 1050 F. before it enters the coiler is by the use of water sprays impinging on the strip between the final stand and the coiler.

When steel of the preferred composition is hot rolled and the finishing and coiling temperatures are properly controlled as described the resulting coil of strip will have a yield strength of at least 50,000 p.s.i. Also, the yield strength and other mechanical properties of the strip will be remarkably uniform throughout the length of the coiled strip.

Although it is desired to use the preferred composition given above, it has been found that steel strip having a yield strength of over 50,000 p.s.i. can be produced from steel falling within the following broader composition limits: C, .10 to .20%; Mn, .40 to 60%; N, .008 to 015%; S, .050% max.; P, .040% max.; Si, .10% max.; Fe and residual impurities, balance.

When working within the broader composition limits it is desirable that when the percentage of any one of the C, Mn, or N is near the lower limit, the percentages of the remaining two should be near their higher limits to insure maximum benefit from the finishing and coiling temperatures.

The coiled strip of steel produced by this invention is eminently suited for use in construction. For example, excellent building purlins can be roll formed from this steel.

While one embodiment of my invention has been shown and described it will be apparent that other adaptations and modifications may be made without departing from the scope of the following claims.

I claim:

1. A method of producing a coiled strip of hot rolled steel having a yield-strength of at least 50,000 p.s.i. in the hot rolled condition, comprising the steps of providing a blank of steel consisting essentially of from .10% to .20% carbon, from .40% to .60% manganese, from .008% to .015 nitrogen, .040% maximum phosphorus, .050% maximum sulphur, .10% maximum silicon, the balance iron and residual impurities, the composition of said steel being further characterized by the limitation that when the percentage of any one of the carbon, manganese, or nitrogen is near the lower limit, the percentages of the remaining two elements should be near their higher limits, hot rolling the blank to form a strip of steel while maintaining the finishing temperature of said trip above 1520" F., rapidly cooling said strip to below 1050 F, and coiling said cooled strip.

2. The method of claim 1 wherein said cooling is provided by water sprays directed against the strip.

3. A carbon steel sheet having .10% to .20% carbon, .40% to .60% manganese, .040% maximum phosphorus, .050% maximum sulphur, .10% maximum silicon, and from 008% to .015% nitrogen, the balance iron and residual impurities, the composition of said steel being further characterized by the limitation that when the percentage of any one of the carbon, manganese, or nitrogen is near the lower limit, the percentages of the remaining two elements should be near their higher limits, having the properties of 50,000 p.s.i. yield-strength in the rolled condition and having been produced by hot rolling a blank of said steel with the finishing temperature having been maintained above 1520 F. and having been rapidly cooled to a temperature less than 1050 F. before coiling.

*4. A coiled strip of carbon steel having .16% to .20% carbon, .40% to .58% manganese, .020% maximum phosphorus, .040% maximum sulphur, .020% maximum silicon, and from 010% to .013% nitrogen, the balance iron and residual impurities, having the properties of 50,000 p.s.i. yield-strength in the hot rolled condition and having been produced by hot rolling a blank of said steel with the finishing temperature having been maintained above '1520 F. and having been rapidly cooled to a temperature less than 1050 F. before coiling.

5. A method of producing a coiled strip of hot rolled steel, having a yield-strength of at least 50,000 p.s.i. in the hot rolled condition, comprising the steps of providing a blank of steel consisting essentially of from .16% to .20% carbon, .40% to .58% manganese, .020% maximum phosphorous, 040% maximum sulphur, .020% maximum silicon, and from .010% to .013% nitrogen, the balance iron and residual impurities, hot rolling the blank to form a strip of steel while maintaining the finishing temperature of said strip above 1520 F., rapidly cooling said strip to below 1050 F. and coiling said cooled strip.

References Cited by the Examiner UNITED STATES PATENTS 2,444,788 7/ 1948 Reichenbach 148l2 2,814,578 11/1957 White 148l2 DAVID L. RECK, Primary Examiner. WINSTON A. DOUGLAS, Examiner. 

1. A METHOD OF PRODUCING A COILED STRIP OF HOT ROLLED STEEL HAVING A YIELD-STRENGTH OF AT LEAST 50,000 P.S.I. IN THE HOT ROLLED CONDITION, COMPRISING THE STEPS OF PROVIDING A BLANK OF STEEL CONSISTING ESSENTIALLY OF FROM .10% TO .20% CARBON, FROM .40% TO .60% MAGANESE, FROM .008% TO .015% NITROGEN, .040% MAXIMUM PHOSPHORUS, .050% MAXIMUM SULPHUR, .10% MAXIMUM SILICON, THE BALANCE IRON AND RESIDUAL IMPURITIES, THE COMPOSITION OF SAID STEEL BEING FURTHER CHARACTERIZED BY THE LIMITATION THAT WHEN THE PERCENTAGE OF ANY ONE OF THE CARBON, MANGANESE, OR NITROGEN IS NEAR THE LOWER LIMIT, THE PERCENTAGES OF THE REMAINING TWO ELEMENTS SHOULD BE NEAR THEIR HIGHER LIMITS, HOT ROLLING THE BLANK TO FORM A STRIP OF STEEL WHILE MAINTAINING THE FINISHING TEMPERATURE OF SAID TRIP ABOVE 1520*F., RAPIDLY COOLING SAID STRIP TO BELOW 1050*F., AND COILING SAID COOLED STRIPS. 